1. Field of the Invention
This invention relates generally to a flooring system in a nuclear reactor, and more particularly, to a floor tile and transition plenum used in a nuclear power plant containment sump strainer system.
2. Description of Related Art
A reactor pressure vessel (RPV), such as in a boiling water reactor (BWR) or pressurized water reactor (PWR) nuclear power plant typically will disperse debris to the containment floor following a design basis Loss of Coolant Accident (LOCA) because such reactors have numerous insulated piping systems, and such piping systems are utilized to transport water throughout the reactor system. Nuclear plant design requires inclusion of plant systems for LOCA mitigation. A LOCA results when high pressure pipe ruptures with such force that large quantities of debris, such as pipe thermal insulation, coatings, concrete and other solids may be dispersed onto the containment floor along with reactor coolant and emergency system coolant pumped into the system to cool the reactor fuel (coolant injection) and containment equipment and structures (containment spray). As a result, the coolant being pumped into the reactor system and containment can cause the LOCA generated debris and other latent debris to be transported along with the coolant to the containment sumps where the emergency pumps take suction through containment sump strainers (or screens). The emergency pumps route the flow external to the containment through heat exchangers and re-introduce it to the reactor and containment spray systems in the post LOCA recirculation mode for long term cooldown of the reactor system. The fallen debris can accumulate on the containment sump strainers and affect the volumetric flow rate of cooling water delivered to the reactor and containment, which in turn, could lead to reactor core overheating.
A conventional approach to the above problem has been to install sump strainers at the containment sump to remove the debris while delivering appropriate amount of volumetric flow rate of water to the reactor following a LOCA. Sump strainers are generally used to remove debris or solids from the fluid present in the containment pool when the fluid is drawn into pump(s) in the Emergency Core Cooling System (ECCS) or the Containment Spray system. Sump strainers may prevent system degradation as the debris is collected at the sump strainers and prevent the debris from distributing throughout the reactor and containment spray systems while operating the post LOCA recirculation mode.
However, sump strainers tend to become clogged by large amounts debris due to small strainer size.
Further, sump strainers typically produce suction at a localized high entrance velocity. Localized high entrance velocities are established where the sump strainer is most proximate to the suction line of the pump, whereas low entrance velocities are established where the sump strainer is more distant from the suction line of the pump. The high entrance velocities may draw more solid debris into contact with the sump strainer causing the portions of the sump strainer experiencing the high entrance velocities to experience higher head loss. As the portion of the sump strainer most proximate to the suction line collects debris, high entrance velocities are established at the portion of the sump strainer that is next closest to the suction line causing that portion to collect debris. This process continues until the entire sump strainer has collected debris in varying quantities, resulting in a build-up of debris on the outer surface of the strainer. Localized high entrance velocities can be detrimental even when solids are not present in the liquid being pumped. For example, high entrance velocities can result in turbulent flow which tends to create greater pressure losses than laminar flow. Any such pressure losses reduce the net positive suction head available to a pump. As the net positive suction head available decreases, pump cavitation may occur. Similarly, localized high entrance velocities may cause vortexing. When a sump strainer is not sufficiently submerged, the vortexing can cause air ingestion which can severely degrade pump performance.